The development of the first known synthetic blue pigment, Egyptian blue (CaCuSi4O10), is believed to have been patronized by the Egyptian pharaohs, who promoted the advancement of pigment technologies for use in the arts. The subsequent quest for blue pigments has a history linked with powerful civilizations, such as the Han Chinese [Han blue (BaCuSi4O10)] and the Maya [Maya blue (indigo intercalated in magnesium aluminosilicate clays)]. Currently used blue inorganic pigments include cobalt blue (CoAl2O4), ultramarine (Na7Al6Si6O24S3), Prussian blue (Fe4[Fe(CN6)]3), and azurite (Cu3(CO3)2(OH)2).
All of these pigments, however, suffer from environmental and/or durability issues. Cobalt is considered to be highly toxic. Ultramarine and azurite are not stable with respect to heat and acidic conditions. Additionally, ultramarine manufacture produces a large amount of SO2 emissions. Prussian blue liberates HCN under mild acidic conditions. Hence, there is a need for inorganic pigments, particularly intensely blue inorganic pigments, which are environmentally benign, earth-abundant and durable.